This invention relates to an apparatus for measuring the torque applied to a driven shaft, particularly to the individual worm conveyor shafts of a multishaft extruder.
Striving for ever-increasing output rates in multishaft worm conveyor extruders requires ever-increasing drive outputs and thus inherently higher input torques for the worm conveyor shafts. Also, the ever-improving process technology makes possible a continuous operation of the machine at almost 100%.
Because of the above requirements, the worm conveyor shafts of current design can be used only with very low safety values as far as their strength is concerned. A breakage of a worm conveyor shaft in a multishaft machine causes not only an interruption in the production, but also results in significant consequential damages to the extrusion part and in the machine drive. Further, the division of the output among several worm conveyor shafts is not uniform. In case of a two-shaft machine with unidirectionally driven, right-pitch worm conveyor shafts, the left-hand shaft--as viewed in the direction of conveyance--is stressed to a greater extent. For this reason a very accurate torque monitoring of each individual worm conveyor shaft is required.
European Patent No. 0 457 086 discloses a contactless measuring of the local torque input in worm conveyor-type extruder machines. For this purpose, in the conveyor housing a plurality of path sensors are arranged which scan the surface of the worm conveyor shaft during its rotation. For an improved scanning, the screw conveyor shafts are provided with notches or grooves. In addition, at the drive clutch, externally of the worm conveyor housing, a separate measuring unit is arranged which delivers a reference pulse from a location of the extruder machine where the shaft is not yet exposed to torsion. Such reference signal is compared and evaluated in a computer with signals generated in the housing. Such a complex and expensive measuring process cannot be used for multishaft worm conveyor machines; even in a single-shaft machine its use is of questionable value when viscous synthetic materials are handled. Also, the prior art system is limited to a monitoring of rotating shafts and therefore the torque of an immobilized (blocked) shaft remains undetected.